Thermal Storage Gets More Solar On The Grid

It’s 4:45 on a sweltering August afternoon, and the rooftop solar panels are starting to lose juice. The sun’s lower angles and that huge cottonwood tree are interfering with the efficient photon-to-electricity transfer.

What is an environmentally conscious — but air-conditioning-loving — homeowner to do?

image via Shutterstock

Peak demand for electricity in the United States typically hits between 4 p.m. and 8 p.m., which doesn’t quite line up with the sun’s schedule. It’s fortunate that the sun is high in the sky during many of the hours when the air conditioning is in demand. But in summer, people tend to need air conditioning during the dinner hour and beyond, when kitchen appliances are whirring, lights are on, and TVs are blaring.

To the rescue comes concentrating solar power (CSP), a technology being tested and deployed by utilities in America’s deserts and southern Spain.

New analysis at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) has found that CSP, with its greater grid flexibility and ability to store energy for as long as 15 hours, can enhance total solar power generation and actually give photovoltaic (PV) systems a greater presence on the grid.

PV panels convert photons from the sun directly into electrons for electricity — and are grabbing real estate on rooftops across the Americas, Europe, and Asia.

CSP technologies use mirrors to reflect and concentrate sunlight onto receivers that collect the sun’s heat. This thermal energy can then be used to drive a steam turbine that produces electricity for utilities.

Thermal Storage Can Even Out the Bumps

Like Edison and Tesla or Dempsey and Tunney, the two major solar energy technologies never meant to play nice. Each had its niche — and its dreams of market share.

Think of power from PV as a roller coaster of highs and lows, and power from CSP, via thermal energy storage, as a gently rolling train.

PV panels and wind turbines contribute electricity to the grid, but without the ability to store that power, they cannot supply the grid after the sun sets, or after the wind dies. Even passing clouds can cause drops in the amount of solar energy that gets on the grid.

Large fossil-fueled and nuclear power plants can’t be quickly stopped or started to accommodate variable energy sources such as solar and wind energy.

CSP can even out these ebbs and flows because it can store power and ramp up output when the amount of direct wind or solar power drops.

Grid Flexibility is the Key

“It all gets down to grid flexibility,” Denholm said. “What sets of grid technologies do you deploy to make the grid respond faster and over a greater range to the input of variable energy such as solar and wind?

“If you can’t respond quickly, you end up potentially throwing away wind and solar energy.

“We know that the more wind and solar you add to the grid, the harder it is to balance the grid and maintain reliability.”

A CSP plant works by heating a heat transfer fluid that is used to boil water to make steam. But because of thermal inertia, by the time that fluid gets through the system’s pipes to the power plant, perhaps 10 or 15 minutes have passed.

When a cloud passes over a PV panel, the drop in energy production is immediate. But because of the 10 or 15 minutes of thermal inertia, a cloud passing over a CSP tower doesn’t cause this immediate drop. Nor is there the immediate surge when sunlight returns.

“The change is more gradual,” Denholm said. “That’s one reason CSP can bring a greater quality to the grid.”

Still, the greater potential for CSP — and for CSP helping PV to expand its role on the grid — is its capacity to store the energy it captures from the sun for several hours, making it a source of reliable energy after the sun sets.

“CSP can fill in that gap in the evening when there’s peak demand for electricity,” Denholm said. “Together, the solar resource can provide all that peak demand. And together they can reduce or eliminate the need to build new power plants for those peak periods.”

At the National Renewable Energy Laboratory (NREL), we focus on creative answers to today's energy challenges. From fundamental science and energy analysis to validating new products for the commercial market, NREL researchers are dedicated to transforming the way the world uses energy.

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